The present inventions relate generally to high-speed digital data communications for use in downhole Measurement While Drilling (MWD) tools. More specifically, the invention relates to a high-speed communications scheme for transferring data between downhole sensors and related electronics. More specifically still, the invention relates to a downhole tool communication system using Quadrature Amplitude Modulation (QAM) or Discrete Multi-Tone (DMT) communication for data streams between or inside drill collars or for communication with external equipment.
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, data relating to the size and configuration of the borehole itself, as well as environmental factors such as temperatures, flow, pressures and the like. The collection of information relating to conditions downhole, which commonly is referred to as “monitoring” or “logging”, can be performed by several methods.
In conventional oil well wireline logging, a probe or “sonde” housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
An alternative method of monitoring and logging is the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing or tripping the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing drilling down time. Halting drilling for monitoring purposes can be extremely costly.
Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement while drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging while drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term MWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
Sensors or transducers typically are located at the lower end of the drill string in MWD systems. While drilling is in progress these sensors continuously or intermittently monitor predetermined drilling parameters and formation data and may transmit information to a surface detector by some form of telemetry. Typically, the downhole sensors employed in MWD applications are positioned in one or more cylindrical drill tool/s that is/are positioned close to the drilling bit. The MWD system then employs a system of telemetry in which the data acquired by the sensors is transmitted to a receiver located on the surface.
There are a number of telemetry systems in the prior art, which seek to transmit information regarding downhole parameters up to the surface. Acoustic telemetry using a mud pulse system is one of the most widely used telemetry systems for MWD applications. The mud pulse system of telemetry creates “acoustic” pressure signals in the drilling fluid that is circulated under pressure through the drill string during drilling operations. The information that is acquired by the downhole sensors is transmitted by suitably timing the formation of pressure pulses in the mud stream. The information is received and decoded by a pressure transducer and computer at the surface.
In addition, when multiple tools may be employed, communication between tools becomes beneficial. Existing methods of communication between tools are often slow and bandwidth limited based on the limited interconnections and noisy environment in close proximity to the drill and other tools. Therefore, what is needed in the art is a wider bandwidth means of communicating between tools without adding additional contacts.